This invention relates generally to the manufacture of circuit boards, more particularly, the surface mounting of integrated circuits (ICs) on substrates including printed circuit boards and other platforms where components are mounted and connected. More particularly, the invention relates to a die feeder device that performs the function of removing each known good die from a sawed wafer disposed on a flexible adhesive film, placing the die on a conveyer, and conveying the die to a pick-up location.
In the mass production of printed circuit boards having a plurality of ICs surface-mounted thereon, automated machinery accept die from feeders and then mount them at a predetermined location on the circuit board. The die feeders have previously taken the form of tape-and-reel-type feeders (tape feeders) which remove the die from a plastic tape and present them to the host automated assembly machine. An example of a reel type tape feeder is shown in U.S. Pat. No. 4,437,232, issued to Araki et al. on Mar. 20, 1984. One advantage of the tape feeders is that they take up a small amount of space during the assembly of the circuit board, thus a large number of tape feeders can be placed adjacent to one another and utilized by a single assembly machine. Additionally, tape feeders are relatively simple in operation and are capable of feeding die reliably at a very rapid pace. The present invention takes the place of these tape feeders and obviates the steps of placing die onto the plastic tape then removing them from the plastic tape. The present invention is capable of performing this function of presenting die serially directly from a wafer without taking up significantly more space than a tape feeder. For this reason, it is called a direct die feeder, or DDF. By reducing the handling of bare die on its journey from wafer to assembly, significant expense reduction is realized, and the opportunity for damage to individual die is reduced, which improves the reliability of the end product.
Because the present invention is intended to replace current tape feeders, it is designed to maintain the narrow form-factor width standard of 80 mm which is standard in the industry. The narrow width enables many direct die feeders of the present invention to be positioned side by side (or along with tape feeders) and thus accessible by a single pick and place machine. By maintaining the narrow form-factor, fewer pick and place machines are needed as well as fewer pick and place stations in the assembly line. This significantly affects the overall cost and time consumed in the production of each unit. Maintaining this narrow profile required many design innovations herein disclosed.
The present invention represents a significant departure from the established practice of packaging dies in carrier tapes, shipping the die in the carrier tapes to a circuit board manufacturer, and then feeding the tapes to pick and place machines. When using the present invention, each die is picked from the wafer and presented directly to the host automated pick and place machine via a conveyor.
The feeder can also replace matrix trays to present parts to the host pick and place machine. The tray feeders represent a considerable expense as they take up a large section of the host machine interface as well as the cost of buying and stuffing trays.
Wafers usable by this invention are provided in a manner which is standard in the industry. A wafer which has been etched to form the micro-electric circuitry thereon is placed on a flexible adhesive film which holds the wafer in place during a sawing (or "dicing") operation. The adhesive film is mounted in a ring which holds the flexible film during removal of the die. Ordinarily, the wafer is "expanded", which means that the flexible adhesive film is stretched after dicing so that individual die spread out from one another. The mounting ring is used to maintain the flexible film in the stretched state. An expanded ring is characterized by an enlarged gap between adjacent dies, which improve the reliability of die extraction and prevents "chip-out" (damage to the picked die or adjacent die caused by scraping the die edges during pick-up).
A machine vision system is employed by the present invention to perform die recognition using the die edges, which requires gaps between the die, permitting them to be perceived by a visual imaging apparatus. The gaps caused by the dicing operation, without further expansion, is often sufficient to enable the chips to be reliably recognized and removed by the present invention, although it is recommended that the wafer be expanded prior to removing die.
Prior attempts at delivering chips directly from wafer to circuit board have either not been successfully implemented or industry has not accepted them. Presumably, lack of interest in the industry resulted from the failure of previous attempts to provide the specific features required by the industry, such as the high speed and small footprint currently made possible by tape feeders.
For example, U.S. Pat. No. 4,876,791 to Michaud et al. discloses an apparatus for removing die from an expanded wafer and placing the die on a substrate. This patent teaches holding the expanded wafer horizontally with the wafer side facing down. A pick and place head is positioned beneath the wafer and above the horizontally-disposed substrate. This configuration permits the pick and place head to remove the die from the wafer and place it on the substrate with minimal movement. However, it suffers from the disadvantage that it is not compatible with existing assembly machinery and requires a significant amount of horizontal real estate. In using the apparatus described in the Michaud et al. patent, the substrate must make a separate stop beneath each wafer and a separate mounting apparatus must be utilized for each chip to be placed on the substrate. The time and expense of using a separate machine for each die installed on the substrate and the space requirements of such an operation make this patent impractical for most assembly operations. Additionally, the horizontal orientation of the Michaud et al. patent is impractical for wafers having a diameter greater than 300 mm due to sagging of the wafer in the center which makes large wafers difficult to handle horizontally.
U.S. Pat. No. 5,671,530 to Combs et al. discloses an apparatus which removes die from an expanded wafer oriented vertically for use in the environment intended for the present invention. The patent to Combs et al., however, suffers from the disadvantages of only being capable of handling one die at a time using pick and place head assembly 104 shown in FIG. 6 of the Combs et al. patent. Pick and place head assembly 104 travels from a pickup location 96 to a transfer location 106. Head assembly 104 cannot return to the pickup location 96 until the circuit board assembly machinery takes the die at transfer location 106. This pick-move-transfer-move-pick cycle is extremely time-inefficient, making the system impractical for many customers who mass produce circuit boards. It is important to understand that the conveyor shown in the Combs et al. patent operates to transport printed circuit boards (referred to as "substrates") during assembly, not to transport bare die to a pickup location as does the conveyor of the current invention.
The Combs et al. patent also suffers from the disadvantage of only being able to present chips in a "flipped" (circuit-down) orientation. There are currently two primary methods of surface-mounting chips onto a substrate or circuit board. The conventional wire-bond or tape-bond methods includes placing the chip on the substrate in the conventional, circuit-up orientation, and electrical connections being made from electrical contacts on the top of the chip to contacts on the substrate using metal wires or tape. The flip-chip method includes forming solder bumps on the art side of the die, then placing the die, circuit-down on the substrate, aligning the solder bumps on the die with metal contacts on the substrate and holding the die in position while electrical and mechanical connection is made by reflowing the solder bumps. The Combs et al. invention is limited by only being capable of presenting chips in the flipped, or circuit-down, orientation.